In packet switching systems, a large number of customers are typically connected to a main packet switching network via digital customer interfaces (also called packet concentrators.) Each such interface accepts packets from the customers via low-speed transmission links and retransmits them to the switching network via a high-speed transmission link.
With the introduction of more complex protocols such as X.25 (CCITT 1976, amended 1980) for defining synchronous transmission of packets between a digital customer interface and individual customers, a problem exists in how to perform the time consuming functions required by such protocols. In particular, a deficiency arises as to how to perform level 2 functions of the X.25 protocol which include the forming of individual data bit signals into bytes, performing the necessary cyclic redundancy checking for errors, performing bit stuffing and unstuffing, and recognizing flag bit patterns. Also, the number of real time processing operations that must be performed to execute illustratively level 3 of the X.25 protocol seriously impacts the ability of existing microprocessors to execute that level for a large number of customers.
In prior art systems, execution of level 2 functions of X.25 is a problem that is solved using individual digital interfaces on a per customer basis. Such per customer requirements have the disadvantages of high cost, substantial complexity, high power consumption, as well as, greater burdens in maintenance. These disadvantages arise because such individual digital interfaces require parallel address and data transfers and are sophisticated, complex, individual circuits requiring large amounts of power.
Multiline protocol formatters, which interface to a number of customers, are known in the art. One example of such a formatter is disclosed in U.S. Pat. No. 3,587,058, T. T. Butler, et al. The Butler formatter is capable of communicating asynchronous data with a plurality of data customer terminals each terminated on a customer interface unit. However, the protocol functions performed by that formatter are much simpler than those required by level 2 of X.25. In addition, the Butler formatter addresses and communicates data, control, and status information with the interface units via parallel buses. Also, the control and status information is communicated in common with data information resulting in a loss of system efficiency during the control of the interface units.